Ivory Apples - Lisa Goldstein

Ivy and her sisters have a secret: their reclusive Great-Aunt is actually Adela Madden, inspired author of the fantasy classic, Ivory Apples. Generations of obsessive fans have searched for Adela, poring over her letters, sharing their theories online, and gathering at book conventions.
It is just a matter of time before one fan gets too close.

So when the seemingly-perfect Kate Burden appears at the local park, Ivy knows that something isn't right. Kate has charmed the entire family, but she is suspiciously curious about Ivory Apples.
And Ivy must protect what she and her Great-Aunt share: magic that is real, untamable, and-despite anyone's desire-always prefers choosing its own vessel.